In the laser structure, crystal defect or dislocation of a waveguide causes electron-hole pairs to make non-radiative recombination therein. Ideally, considering the laser function, the dislocation density in the waveguide should be 104/cm2 or less. However, in the present situation, the dislocation density can not be reduced less than 106/cm2 by using a vapor phase epitaxial growth (MOCVD and HVPE) or by using a repeated ELOG (Epitaxial lateral overgrowth) because the waveguide is grown on a heterogeneous substrate, such as sapphire substrate or SiC substrate.
The thickness of the epitaxial layer should be 100μ or less to form the epitaxial layer on a sapphire substrate or a SiC layer without crack by the vapor phase growth. Moreover, a nitride crystal is formed on C-plane as an epitaxial growth face on the heterogeneous substrate such as the SiC substrate or the sapphire substrate by a vapor phase growth, which leads the active layer of the laser structure formed thereon to have a spontaneous polarization or piezo polarization causing a deterioration of the performance such as red shift of light emitting, recombination degradation and increase of the threshold current.
Moreover, in case that the active layer deposited on the heterogeneous substrate is formed by a quantum well layer containing In, the active layer is influenced by the crystal condition of the n-type nitride layer as a base, therefore it is necessary to form an undoped GaN layer or super lattice structure layer as the base. On the other hand, when the p-type nitride layer is deposited on the active layer containing In, a protective layer is preferably formed to avoid the active layer degradation by evaporation of In therefrom. When the protective layer for GaN or AlGaN layer is formed, the protective layer is formed at a temperature from 800° C. to 900° C. which is lower than that at formation of the active layer. Accordingly, a resultant nitride layer will be in the form of amorphous, which influences the crystal condition of the optical guide layer and p-type clad layer formed thereon.
To summarize the above, there is a limitation to form layers of the laser device by a vapor phase growth. Moreover, regarding the light emitting diode, in case that the higher luminance and higher output are required, the crystal dislocation of the substrate and the intermediate layer will be a serious problem.